Waveguide window

ABSTRACT

A window for a waveguide for a beam of electromagnetic radiation. The window includes a layer of a material capable of allowing electromagnetic radiation to pass therethrough and a support on each side of the layer. Each support defines a passage for electromagnetic radiation. Further, the layer is provided with at least one recess formed in the periphery thereof.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to a window, for example a window for a waveguidefor a beam of electromagnetic radiation.

2. Discussion of the Background

Electromagnetic radiation, and more particularly microwave radiation, isgenerated in gyrotrons and other high power microwave sources. A beam ofthe radiation is generated and this passes along a waveguide to an exitport. The waveguide comprises a passage, generally of circularcross-section, having a window extending across it. The beam ofelectromagnetic radiation passes through the window. The window is usedfor the protection of the electromagnetic sources or detectors fromenvironmental factors. The window may be made of a variety of materialssuch as sapphire, diamond and the like. Such windows are generallyplanar, although curved profiles have been suggested.

SUMMARY OF THE INVENTION

According to the present invention, a window for a waveguide for a beamof electromagnetic radiation, comprises a layer of a material capable ofallowing electromagnetic radiation to pass therethrough and a support oneach side of the layer, each support defining a passage forelectromagnetic radiation, and the layer being provided with at leastone recess formed in the periphery thereof.

Preferably a plurality of recesses are formed in the periphery of thelayer, those recesses preferably being evenly spaced around theperiphery. The number of recesses will vary according to the nature ofthe layer and will typically exceed six.

The depth of the recess or recesses into the layer will vary accordingto the nature of the material from which the layer is made, the size andshape of the layer and other such factors. For example, where the layeris disc-shaped, the depth of the recess or recesses may be chosen toreduce the peak level of hoop stresses in the layer.

In one form of the invention, each support is located inside theperiphery of the layer and the or each recess has a depth equal to orless the distance of the supports from the periphery.

It is preferred that each recess is free of sharp corners, e.g. has arounded end which extends into the layer.

Each support may take the form of a ring having a flange which bearsagainst a surface of the layer. The flanges will generally extendoutwards and away from the passage defined within the supports. Tominimise expansion of the flanges, a retaining ring may be providedaround the outwardly extending flanges. The retaining ring may be madeof a material such as molybdenum.

The supports will generally be bonded to a surface of the layer by meansof a braze or diffusion bond. The layer may take any suitable shape suchas rectangular, but will generally be disc-shaped.

The material from which the layer is made will typically be sapphire,diamond, germanium, zinc selenide, silicon, doped silicon, siliconnitride, aluminium nitride or boron nitride. The window is preferablymade of diamond. Such diamond is preferably produced using chemicalvapour deposition (CVD).

The invention extends to a layer for use in a window as described above,such layer being of a material capable of allowing electromagneticradiation to pass therethrough and being provided with at least onerecess formed in the periphery thereof.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a plan view of an embodiment of a window of the invention, and

FIG. 2 is a section along the line 2—2 of FIG. 1.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

An embodiment of the invention will now be described with reference toFIGS. 1 and 2. Referring to the drawings, there is shown a windowcomprising disc-shaped layer 10, preferably made of CVD diamond, havingopposite major surfaces 12, 14 and a peripheral edge 16. The layer 10 ismounted between opposed supports or cuffs 18, 20. Each cuff comprises aring section 18 a, 20 a and an outwardly extending flange 18 b, 20 b.The flange 18 b bears against the surface 12, whilst the flange 20 bbears against the surface 14. The flanges 18 b, 20 b, are bonded to thesurfaces 12, 14 respectively, of the CVD diamond layer 10, by means of adiffusion bond. The bonding material will be chosen to suit theapplication. Examples of suitable materials are aluminium and gold andalloys thereof.

A passage 22 is defined within the cuff 18 and a passage 24 is definedwithin the cuff 20. The cuff 18 is in alignment with the cuff 20 so thatthe passages 22 and 24 have no discernible discontinuity.

Retaining rings 26 and 28 are provided around the outwardly extendingflanges 18 b, 20 b. The retaining ring preferably has a coefficient ofexpansion which is comparable to that of the layer 10 and lower thanthat of the cuffs 18, 20. An example of a suitable material ismolybdenum.

The cuffs 18, 20 will typically be made of a metal such as Inconel. Theattachment of a CVD diamond layer to a metal cuff presents severalproblems. First, a compatible braze or diffusion bond must be chosenhaving adequate strength and high temperature creep resistance towithstand bakeout. Secondly, the metal cuffs must be chosen having alinear thermal expansion match to the diamond window. While primarilyfor the high temperature bonding process, this requirement also affectsstresses encountered in subsequent vacuum bakeout treatments which cancause cumulative work hardening effects in some metal joints leadingultimately to fracture. Diamond has one of the lowest thermal expansioncoefficients of any material and any metal cuff will expand considerablymore than diamond over the 0-1000° C. temperature range causing mismatchstresses. Finally, the thermal variations from centre to edge in the CVDdeposition process can lead to hoop stresses across the layer 10. Themagnitude of hoop stresses can sometimes be sufficient to promote radialcracks propagating in from the edge of the disc. It is important tocontain these cracks if the disc is to be considered for vacuum windowapplications. Even if the cracks are not present initially, the presenceof the stresses can lead to cracking later during the lifetime of thewindow.

A plurality of recesses or slots 30 are provided around the periphery 16of the disc-shaped layer 10. Each slot 30 extends into the layer 10 fromthe periphery 16 and each slot has a rounded end or base 32. The slots,it has been found, give rise to the following advantages:

1. The slots facilitate the use of alignment pins and fixtures, whichare simpler and lower in cost, directly against the cuffs 18, 20 toensure that accurate concentric alignment of the two cuffs and theirbonding to the layer is achieved. A typically error of 0.3 nm in thisregard can be reduced to better than 0.05 mm.

2. The use of slots reduces the peak level of hoop stresses that existin the layer 10 particularly in the region between the cuffs 18, 20 andthe edge 16. This in turn reduces the likelihood of uncontrolled radialcracks extending in from the edge 16. The number of slots requiredpreferably exceeds six and each slot should have a depth equivalent tothe region between the cuffs 18, 20 and the edge 16.

3. The thermal mismatch stresses between the cuffs 18, 20 and the layer10 are reduced since the slotted layer is effectively more compliant inthe region of the bonding. Twisting forces which may exist in the outerperiphery of the layer 10 are reduced. The likelihood of bond failureduring repeat thermal cycling is also reduced.

4. The slots can be used to modify the liquid cooling around the edge ofthe window in order to derive an advantage in the heat transfer.

The window described above may be mounted in a waveguide using methodsand techniques known in the art.

What is claimed is:
 1. A window for a waveguide for a beam ofelectromagnetic radiation, comprising: a layer capable of allowingelectromagnetic radiation to pass therethrough, the layer having atleast one recess formed in a periphery portion thereof; and a pluralityof supports disposed on side portions of the layer, respectively, theplurality of supports defining a passage for electromagnetic radiation,wherein the plurality of supports is located inside of the peripheryportion of the layer and the at least one recess extends from theperiphery portion to the plurality of supports at most.
 2. A windowaccording to claim 1, wherein the at least one recess comprises aplurality of recesses formed in the periphery portion of the layer.
 3. Awindow according to claim 2, wherein the plurality of recesses areevenly spaced around the periphery portion.
 4. A window according toclaim 1, wherein the at least one recess comprises at least sixrecesses.
 5. A window according to claim 1, wherein the at least onerecess has an inner surface portion which is free of sharp corners.
 6. Awindow according to claim 5, wherein the at least one recess has arounded end which extends into the layer.
 7. A window according to claim1, wherein the plurality of supports each have a form of a ring having aflange which bears against a surface portion of the layer.
 8. A windowaccording to claim 7, wherein the flanges of the plurality of supportsextend outwards and away from the passage defined within the pluralityof supports.
 9. A window according to claim 8, further comprising aretaining ring provided around the flanges.
 10. A window according toclaim 9, wherein the retaining ring comprises molybdenum.
 11. A windowaccording to claim 1, wherein the layer comprises a material selectedfrom the group consisting of sapphire, diamond, germanium, zincselenide, silicon, doped silicon, silicon nitride, aluminum nitride andboron nitride.
 12. A window according to claim 1, wherein the layercomprises CVD diamond.
 13. A window according to claim 1, wherein thelayer is disc-shaped.
 14. A window for a waveguide for a beam ofelectromagnetic radiation, comprising: a layer capable of allowingelectromagnetic radiation to pass therethrough, the layer having atleast one recess formed in a periphery portion thereof; and a pluralityof supports disposed on side portions of the layer, respectively, theplurality of supports defining a passage for electromagnetic radiation,wherein the at least one recess has an inner surface portion which isfree of sharp corners.
 15. A window according to claim 14, wherein theat least one recess comprises a plurality of recesses formed in theperiphery portion of the layer.
 16. A window according to claim 15,wherein the plurality of recesses are evenly spaced around the peripheryportion.
 17. A window according to claim 14, wherein the at least onerecess comprises at least six recesses.
 18. A window according to claim14, wherein the at least one recess has a rounded end which extends intothe layer.
 19. A window according to claim 14, wherein the plurality ofsupports each have a form of a ring having a flange which bears againsta surface portion of the layer.
 20. A window according to claim 19,wherein the flanges of the plurality of supports extend outwards andaway from the passage defined within the plurality of supports.
 21. Awindow according to claim 20, further comprising a retaining ringprovided around the flanges.
 22. A window according to claim 21, whereinthe retaining ring comprises molybdenum.
 23. A window according to claim14, wherein the layer comprises a material selected from the groupconsisting of sapphire, diamond, germanium, zinc selenide, silicon,doped silicon, silicon nitride, aluminum nitride and boron nitride. 24.A window according to claim 14, wherein the layer comprises CVD diamond.25. A window according to claim 14, wherein the layer is disc-shaped.